Sheet detecting apparatus and image forming apparatus

Abstract

A sheet detecting apparatus including: a sheet detecting member having a plurality of abutment surfaces in a peripheral direction thereof, the sheet detecting member being rotated by a conveyed sheet abutting against one of the plurality of abutment surfaces; a detection portion which operates in association with the sheet detecting member; a sensor which generates a signal based on a position of the detection portion; and an urging portion which generates an urging force for positioning the one of the plurality of abutment surfaces of the sheet detecting member in a waiting position in which the leading edge of a sheet conveyed by a conveying portion abuts against the one of the plurality of abutment surfaces, and thereafter the urging portion switching the urging force to an urging force for positioning, in the waiting position, another one of the plurality of abutment surfaces against which a succeeding sheet abuts.

What is claimed is: 
     
       1. A sheet detecting apparatus which detects a sheet conveyed by a conveying portion which conveys the sheet, the sheet detecting apparatus comprising:
 a sheet detecting member including a plurality of abutment surfaces against which a leading edge of the sheet conveyed by the conveying portion abuts, the plurality of abutment surfaces being formed in a peripheral direction of the sheet detecting member, the plurality of abutment surfaces of the sheet detecting member being pressed by the conveyed sheet to rotate from a waiting position; 
 a sensor which generates a signal based on a position of the plurality of abutment surfaces; 
 a shaft connected to the sheet detecting member to support the sheet detecting member in a rotatable manner; 
 a first rotary member connected to the shaft and configured to rotate around a first rotation center; 
 a second rotary member connected to the first rotary member and configured to rotate around a second rotation center, wherein if N is a number of the plurality of abutment surfaces, one revolution of the second rotary member around the second rotation center makes 1/N revolution of the first rotary member around the first rotation center; and 
 a biasing portion configured to bias the second rotary member to be positioned at a predetermined position in a rotation direction of the second rotary member around the second rotation center so as to position one of the plurality of abutment surfaces at the waiting position. 
 
     
     
       2. A sheet detecting apparatus according to  claim 1 , wherein the biasing portion comprises an urging spring, wherein one end of the urging spring is positionally-fixed and another end of the urging spring is connected to a connecting portion arranged eccentrically from the second rotation center of the second rotary member. 
     
     
       3. A sheet detecting apparatus which detects a sheet conveyed by a conveying portion which conveys the sheet, the sheet detecting apparatus comprising:
 a sheet detecting member including a plurality of abutment surfaces against which a leading edge of the sheet conveyed by the conveying portion abuts, the plurality of abutment surfaces being formed in a peripheral direction of the sheet detecting member, the sheet detecting member being pressed by the conveyed sheet to rotate; 
 a sensor which generates a signal based on a position of the plurality of abutment surfaces; and 
 a positioning portion which positions one of the plurality of abutment surfaces of the sheet detecting member in a waiting position in which the leading edge of the sheet conveyed by the conveying portion abuts against the one of the plurality of abutment surfaces, 
 wherein the positioning portion comprises:
 a first gear connected to a rotary shaft of the sheet detecting member; 
 a second gear meshed with the first gear, wherein if N is a number of the plurality of abutment surfaces, one revolution of the second gear makes 1/N revolution of the first gear; and 
 an urging spring having one end positionally-fixed and another end connected to a connecting portion arranged offset with respect to a rotation center of the second gear. 
 
 
     
     
       4. An image forming apparatus, comprising:
 a sheet detecting apparatus as recited in  claim 1 ; and 
 an image forming portion which forms an image on a sheet sent from the sheet detecting apparatus. 
 
     
     
       5. An image forming apparatus according to  claim 4 , wherein the image forming portion starts an image formation operation for forming an image on a sheet conveyed based on a signal from a sensor. 
     
     
       6. An image forming apparatus according to  claim 4 , wherein the biasing portion comprises an urging spring, wherein one end of the urging spring is positionally-fixed and another end of the urging spring is connected to a connecting portion arranged eccentrically from the second rotation center of the second rotary member. 
     
     
       7. An image forming apparatus, comprising:
 a sheet detecting apparatus as recited in  claim 3 ; and 
 an image forming portion which forms an image on a sheet sent from the sheet detecting apparatus. 
 
     
     
       8. An image forming apparatus according to  claim 7 , wherein the image forming portion starts an image formation operation for forming an image on a sheet conveyed based on a signal from a sensor. 
     
     
       9. A sheet detecting apparatus according to  claim 1 , wherein the biasing portion connected to a part of the second rotary member located offset with respect to the second rotation center of the second rotary member to hold the second rotary member at the predetermined position in the rotation direction and to position one abutment surface of the plurality of abutment surfaces in the waiting position where the leading edge of the conveyed sheet abuts the abutment surface. 
     
     
       10. A sheet detecting apparatus according to  claim 1 , wherein the biasing portion comprises
 an urging spring which provides the sheet detecting member with an urging force for positioning the one of the plurality of abutment surfaces in the waiting position, the urging spring being connected to the second rotary member in such a manner that, when a rotation of the first rotary member along with a rotation of the sheet detecting member is transmitted to the second rotary member to rotate the second rotary member by a predetermined angle, a state in which the urging spring exerts an urging force for positioning the one of the plurality of abutment surfaces to the waiting position is changed to a state in which the urging spring exerts an urging force for positioning, in the waiting position, another one of the plurality of abutment surfaces against which a succeeding sheet abuts. 
 
     
     
       11. A sheet detecting apparatus according to  claim 1 , wherein the first rotary member is a first gear, and
 the second rotary member is a second gear meshed with the first gear.